AVAudioSession setCategory not working

问题

I have a video capturing app and I want to be able to play background music while recording audio+video.

I can accomplish this if I set the AVAudioSession category to PlayAndRecord in didFinishLaunchingWithOptions. However, this causes a glitch in the audio whenever the view with the camera enters or exits the foreground, and its apparently impossible to get rid of: https://forums.developer.apple.com/message/74778#74778

I can live with the glitch if it just happens when I start/stop recording video, but that means I need to change the AVAudioSession category from Ambient to PlayAndRecord when a button is pressed, and this also seems to not be possible.

How do I change the AVAudioSession category?

I tried this:

[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayAndRecord
                                 withOptions:AVAudioSessionCategoryOptionMixWithOthers
                                       error:nil];

But the background music stops when the recording starts.

Following this post: AVCaptureSession and background audio iOS 7

I tried deactivating the session then setting the category:

AVAudioSession *session = [AVAudioSession sharedInstance];
[session setActive:NO withOptions:AVAudioSessionSetActiveOptionNotifyOthersOnDeactivation error:&error];
[session setCategory:AVAudioSessionCategoryPlayAndRecord
         withOptions:AVAudioSessionCategoryOptionMixWithOthers | AVAudioSessionCategoryOptionDefaultToSpeaker
               error:nil];
[session setActive:YES error:&error];

I also tried listening for the AVAudioSessionRouteChangeNotification and setting the category in that handler after deactivating the session. But the background music always turns off when the app starts recording.

Why can't I change the AVAudioSession category?